In the 1967 mind set the luxuary appointments, (upscale interior, extra exterior trim, p/steering, etc), is what made up a "premium" model.
Accessories like A/C were still a expensive novalty, power brakes ... nice to have but not necessory, The new LA 318 was a great engine. AM/FM radio stations were still scarce.
It was just a marketing tool of the time. Believe it or not .... The car companys were trying to make money ...... Folks who were looking for "premium" models would be more likely to add options. Start with a lower initial cost ....... sell options ....= profit. See how it works ......?